Reduction of aqueous thiosulfate solutions containing formate with carbon monoxide

ABSTRACT

Sodium, potassium, or ammonium thiosulfate is converted in one stage to the corresponding formate and H2S by heating the thiosulfate to an elevated temperature above 475*F. in the presence of carbon monoxide, steam, and one of the following: sodium, potassium, and ammonium formates.

Umted States Patent 1 1 3,7 M691 Yavorsky et al. [45] Febq2i7, 1973 54] REDUCTION OF AQUEOUS THIOSULFATESOLUTIONS [5 6] References Cited CONTAINING FORMATE WITH CARBON MONOXIDE UNITED STATES PATENTS k m1. ..23/115 x [75] Inventors. Paul M. Yavorsky, Monongahela, 3584'042 6/197] Y y V p Everett Gorin b rg i of 3,592,850 7/1971 Mazzacco et a1", .,...23/ll5 X OTHER PUBLICATIONS [73] Assign: Consolidation Coal Company N Goliath et al., Mech. of Reduction of Sulfur Dioxide V by Formic Acid Am Chemica Scand., v. 16, N0. 3, [22] Filed: April 12, 1972 1962, pp- 570-574. v

[21] f 243259 Primary Examiner-Vivian Garner Related U.S Applicafion Data Attorney-D. Leigh FOWlCl', Jr. et al.

[60] Division of Ser. No. 128,750, March 29,1971, Pat. 57 ABSTRACT No. 3,687,614, which is a continuation-in-part of Ser. 1 No. 879,224, Nov. 24, 1969, Pat. No. 3,584,042, Sodium, potassium, or ammonium thiosulfate is conwhich is a continuation-in-part of Ser. No. 667,479, verted in one stage to the corresponding formate and P 1967, abandoned- H S by heating the thiosulfate to an elevated tempera- V I ture above 475F. in the presence of carbon mon0x-- [52] US. Cl. "260/542, 23/63, 23/115, i steam, and one of the following: sodium, i

' 23/181 um, and ammonium fol-mates. [51] Int. Cl ..C07c 51/00 [58] 7 Claims, 1 Drawing Figure Field oi Search ..260/542; 23/63, 181 1 v l REDUCTION OF AQUEOUS TIIIOSULFATE SOLUTIONS CONTAINING FORMATE WITH CARBON MONOXIDE CROSS-REFERENCES TO RELATED APPLICATIONS This application is a division of application, Serial No. 128,750, filed Mar. 29, 1971, now U.S. Pat. No. 3,687,614 which is a continuation-in-part of application, Ser. No. 879,224, filed Nov. 24, 1969, now U.S.

Pat. No. 3,584,042. Application, Ser. No. 879,224 wasitself a continuation-in-part of application, Ser. No. 667,479, filed Sept. 13, 1967 (now abandoned).

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the regeneration of spent absorbent used in the removal of sulfur dioxide from gases containing sulfur dioxide, such as flue gases. The absorbent is sodium, potassium, or ammonium formate in the liquid state.

2. Description of the Prior Art In U.S. Pat. No. 3,584,042, cited above, there is described a two-stage process for regenerating spent absorbent used for removing SO from flue gas. The absorbent, which is sodium, potassium, or ammonium formate in a liquid state, absorbs SO by reacting with it essentially in accordance with the following Equation 1: Equation 1. 2 MCOOH 2 S M 8 0 2 CO H O where M is Na, K, or NH, The two-stage process of regeneration effects the reduction of M 8 0 to MCOOH and H,S in two successive reactions essentially in accordance with the following Equations 2 and 3: Equation 2. M 0 4 MCOOH 3 M CO 2 I-I,S

CO Equation 3. M CO 2 CO +H O 2 MCOOH C0 The foregoing Equations 2 and 3 represent somewhat simplified expressions of what actually occurs. While they do represent the principal conversions, there are side" reactions, namely: Equation 4. 4 MCOOH M S O 4 MI-ICO 2 MSl-l Equation 5. 4 MCOOH M S O 3 CO H O In U.S. Pat. No. 3,592,850, there is described a three-stage process for regenerating the abovedescribed spent formate absorbent which provides for the conversion of any byproduct MSI-l and M 5 (see Equations 4 and 5) as well as the conversion of the principal products. An additional stage is provided between the two reduction stages (see Equations 2 and 3). In this additional stage, CO, and 11,0 (steam) are passed through the product from the first reduction stage, to effect stripping of H,S,and the two reactions expressed in the following Equations 6 and 7: I Equation 6. MSI-l C0; H 0 MHCO 11 Equation 7. M,S C0, H 0 M CO 11,8

SUMMARY OF THE INVENTION In accordance with the present invention, a process is provided for regenerating, in one stage, partially spent formate absorbent used to remove SO, from S0,-

M CO 2 M 8 containing gases. That process comprises passing a CO- containing gas (together with steam, if the absorbent contains no water or insufficient water) through the partially spent absorbent at a temperature above 475F., preferably in the case of aqueous solutions, between 500 and 600F. and at a pressure above 500 psi, preferably between 750 psi and 1500 psi. Higher temperatures are used if the absorbent is in a molten 1 state; and the reducing gas also preferably contains hydrogen in such cases.

One of the keys to successful conduct of regeneration in one stage is the conversion of the by-product MSI-l (see Equation 4) to MOOCI-l and H S, as follows: Equation 8. MSH C0 [-1 0 MOOCH H S This reaction is believed to be new.

The feasibility of single-stage (one-zone) regeneration as depicted by the reaction Equation 9. K S O 6 C0 3 H O 2 KOOCH 4 C0 2 ms was experimentally tested on synthetic spent aqueous potassium formate at a set of conditions known to be collectively suitable for all of the following reactions: Equation 10. 4 KOOCI-l K S O H O 4 KHCO 2 KSH Equation 1 l. KHCO, KSH K CO H S Equation 12. [($11 C0 H O KOOCH H 8 Equation 13. K CO 2 C0 H O 2 KOOCH C0 Carbon monoxide, in excess of that required by reaction of Equation 13 was supplied continuously to the thiosulfate-formate solutions: (1) to provide for the ultimate synthesis of formate from the reduction and stripping product, and (2) to act as a vehicle to remove H S from the reactor. In one of the runs, CO was also fed to assist the intermediate reaction of Equation 11 by, in effect, reducing the decomposition of bicarbonate to carbonate at the reaction temperature.

The results of the experimental demonstration of single-stage regeneration may be summarized as follows: All of the sequential reactions of Equations 10 to 13 inclusive, involved in regeneration of formate from thiosulfate, occurred simultaneously in a single-stage reactor at 536F. with an inlet CO partial pressure of 580-700 psia. For two hours residence time, the reduction of thiosulfate to sulfides was complete (99.7 percent), the stripping of H 5 from the intermediate KSH was 87 percent complete and synthesis of formate proceeded to 73.5 percent of total regeneration of all potassium into the formate salt. The pressure of added CO definitely assisted stripping, making K CO available for final regeneration to formate by .CO reduction. Complete sulfur recovery as H 8. and. complete regeneration of formate can likely be obtained by improved gas-liquid contacting in the single-stage reactor because the slower, overall rate-controlling reactions are gas-liquid mass transfer controlled.

The apparatus used for the single-stage regeneration I runs consisted of a magnetically stirred, one-liter, batch-loaded autoclave provided with the necessary temperature and pressure controls. The unit included facilities for continuous pressurized gas feed and monitored gas withdrawal, as well as a condenser for returning water back to the pool of liquid in the autoclave. Operationally, the gas flow and stirrer were shut off at the end of the reaction period and the reactor allowed to cool to room temperature. The autoclave was then depressurized, opened, and the product liquid analyzed.

A11 pertinent conditions and results appear in Table I. The major controlled difference between the two runs is that pure CO was fed in No. 1 run, whereas CO and CO were fed in No. 2 run. The synthetic spent formate feed had 33 percent more formate than required for the stoichiometric reduction of the thiosulfate to assure the potential of complete reduction. Also, the solution fed to Run No. 2 contained more water to make sure that sufficient water was retained for consumption as per the reaction of Equation 13.

Conditions Run No. 1 'Run No. 2 Temperature, F. 536 536 Total Pressure, psig 960 1175 Est.Steam Pressure, psia 260 400 Feed Gas at 410 SCFH 100% C0 25% CO,:75% CO Reaction Time, hr. 2.00 2.00 Stirrer Speed, rpm. 1200 1200 Feed Composition, gm

1(,S,O, 96.00 96.00 KOOCH" 224.00 224.00 11,0 80.00 173.00 Total Charge Weight 400.00 493.00 Product Analysis, gm

KSH 28 .99 7.77 14,5,0, 1.01 0.27 14,80, 1.14 0.89 K,SO, 1.14 1.04 Kl-ICO, 29.16 61.76 K,CO, 46.43 15.36 KOOCH 190.88 227.03 11,0 59.65 129.88 Actual Product Weight 358.40 444.00 H,S-Off 21.86 28.93 Product S Distribution, %S as 11,8 60.1 87.4 KSH 37.6 11.1 K,S,O; 1.0 0.3 K,SO, 0.7 0.6 K,SO 0.6 0.6 Sulfur Balance (out/in) 105.8 96.2 Results Reduction of 105,0, 99.0 99.7 Regeneration of the ReductiomConsumed KOOCH 80.3 101.8 91: Kin Product as KOOCl-I 61.8 73.5

By difference, via S-balance. By difference. via K-balance. By difference, via Mass-balance.

" 167.9 gm KOOCH requirement for complete reduction of K,S,O,.

DESCRIPTION OF DRAWING For a better understanding of our invention, its objects and advantages, reference should be had to the accompanying drawing in which is shown a schematic flowsheet of the process of our invention.

Referring to the drawing, So -containing gas is introduced into the bottom of a scrubber 10 through a conduit 12 while the selected formate (preferably concentrated aqueous potassium formate, e.g. 70-75%) is fed into the top of the scrubber through a conduit 14. The scrubber may be any conventional gas-liquid scrubbing tower designed to assure contact of the SO,- containing gas at elevated temperatures with the selected formate in a liquid state. We prefer to use a jiggling bed of marbles through which the gas and liquid pass in countercurrent flow relationship. The temperapotassium formate is the absorbent. This temperature range has the advantage of eliminating the need for reheat of the scrubbed gases when they are released to the atmosphere. The scrubbed gas, freed of S0,, or substantially so, is discharged through a stack 16 as clean stack gas.

The relative amounts of SO -containing gas and formate passing through the scrubber are regulated to provide for considerable excess of the formate, so that less than 25 percent by weight of the formate is converted to the thiosulfate. Accordingly, the major constituents of the effluent liquid stream leaving the bottom of the scrubber through conduit 18 are aqueous potassium formate and potassium thiosulfate. These are circulated through a filter 20 by a pump 22 to the top of a stirred Formate Regenerator vessel 24 wherein the thiosulfate is reduced in a single stage to the formate by reaction with excess formate and with CO introduced through a conduit 26 into the bottom of the vessel 24. Additional water may be added through a conduit 27 if necessary. The temperature maintained in the Regenerator is about 540F., and the pressure held at about 1,000 psig. The residence time is about 2 hours. The gaseous product composed of H 8 and CO is discharged through a stack 28, while the regenerated aqueous formate is recycled to the scrubber through the conduit 14 after suitable adjustment of its concentration.

According to the provisions of the patent statutes, we have explained the principle, preferred construction, and mode of operation of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specifically illustrated and described.

We claim:

1. The process which comprises contacting a mixture of a thiosulfate selected from the group consisting of sodium, potassium and ammonium thiosulfates and a formate having the same cation as said thiosulfate with carbon monoxide and water at a temperature above 475F. and a pressure above 500 psi, whereby H S, formate, carbonate, and hydrosulfide are obtained in the product.

2. The process according to claim 1 wherein H is added to the CO.

3. The process according to claim 1 wherein CO is added to the CO.

4. The process according to claim 1 wherein said mixture is in aqueous solution.

5. The process according to claim 4 wherein the temperature is between 500 and 600F. and the pressure is between 750 and 1,000 psi.

6. The process according to claim 5 wherein the thiosulfate and the formate are the potassium salts.

7. The process which comprises reacting MSl-l WITH CO and 11 0 at a temperature above 475 F and at a pressure above 500 psi, to thereby make MOOCI-I and ms, where M is Na, K, or NH 

2. The process according to claim 1 wherein H2 is added to the CO.
 3. The process according to claim 1 wherein CO2 is added to the CO.
 4. The process according to claim 1 wherein said mixture is in aqueous solution.
 5. The process according to claim 4 wherein the temperature is between 500* and 600*F. and the pressure is between 750 and 1,000 psi.
 6. The process according to claim 5 wherein the thiosulfate and the formate are the potassium salts.
 7. The process which comprises reacting MSH WITH CO and H2O at a temperature above 475* F and at a pressure above 500 psi, to thereby make MOOCH and H2S, where M is Na, K, or NH4. 